I expect this is the classic P&O weekender to Zeebrugge. Excursions are provided both to Brugges and Brussels but I beleive the prices are excessive. We done a similar cruise earlier this year and found the 5 euro return fare on the train to Brugges very good value. A tram runs along the sea front from outdside the dock gates to Blankenberge and also to Zeebrugge Strand station where there is a train to Brugges every hour(The journey takes 25 minutes). True, we didn't have a guide but we didn't have a rigid timetable either and took Brugges and all it had to offer at our leiure. Enjoy the history, the cafe culture and the chocolate.

Unless you want to visit Bruges (assuming you are going to Zeebrugge and that this is only a 2 night cruise) you could stay on board and spend the time enjoying the ship. If you board one afternoon and then spend all the next day out on an excursion you will have very little time to enjoy the ship. If you are first time cruisers you could enjoy a good lunch in the main restaurant and see what else is going on. When a lot of passengers are ashore it is a great time to explore the ship. We have been on Oriana twice - the first time was on a 3 night cruise - and we loved it.